Section 15. The bonds, notes and certificates of indebtedness of an operating agency, in the absence of an express recital to the contrary on the face thereof, shall constitute negotiable instruments for all purposes. They may be payable from the income of the agency or constitute a general obligation thereof, may be sold at not less than par, at public or private sale, may mature at such time or times, may be secured in such manner, may provide for such rights and remedies upon their default, may contain such other covenants, terms and conditions not inconsistent with law, may be executed by such officers, and may be issued with or without the corporate seal, all as may be authorized either by vote of the agency or by the officer or officers to whom the power to determine any or all the matters set forth in this sentence may be expressly delegated by vote of such agency. The engraved or printed facsimile of the seal of an agency on its bonds, notes or certificates of indebtedness shall have the same validity and effect as if such seal were impressed thereon. Whenever a bond, note or certificate of indebtedness is required to bear the signatures of two or more officers, it shall be sufficient if the signature of any one of such officers upon such instrument is a written signature and the remaining signature or signatures are engraved, printed or stamped facsimile signatures; provided, that each officer whose facsimile signature appears on such instrument has, by a writing bearing his written signature and filed in the office of the secretary of the agency, authorized the officer whose written signature appears on such instrument to cause such facsimile to be placed thereon. The facsimile signature of any officer so engraved, printed or stamped thereon shall have the same validity and effect as his written signature. In case any officer whose signature or a facsimile thereof appears on any notes, bonds or coupons shall cease to be such officer before the delivery of such notes or bonds, such signature or facsimile shall nevertheless be valid and sufficient for all purposes as if he had remained in office until such delivery.

The bonds, notes and certificates of indebtedness of an operating agency issued under this chapter and the interest thereon shall be exempt from taxation, with respect to principal and income. The bonds of such an agency issued under this chapter shall be legal investments for the deposits and the income derived therefrom of savings banks, for the trust funds of trust companies, for the capital and other funds of insurance companies, and for funds over which the commonwealth has exclusive control.